Human-induced Changes of Organic Carbon Storage in Soils of China

records (Fig. 3C and D) at Jingbian show four stepped increases in sandsized particle content. The late Pliocene red clay (below L33) contains few sand particles, indicating that the dust was transported in suspension, mainly from a remote source. From ~2.6 to ~1.2 Ma, sand content in interglacial soils remains low, whereas it varies generally between 18% and 25% in glacial loess, except for the case of L15 and L16. This suggests that during glacial periods, the desert environment advanced to a location no more than 200 km from the present northern margin of the Loess Plateau. In the part of the section deposited between ~1.2 and ~0.7 Ma, sand content increases to ~12% in soils and to ~43% in loess, with a substantial increase in >125 μm particles, implying a large-scale advance of the desert margin during both glacial and interglacial times. Throughout, material deposited in the interval ~0.7-0.2 Ma, >63 μm particles range from ~30% in soils and ~55% in loess units, with the >125 μm particles exceeding 8%. This suggests that the distance between the Loess Plateau and the present desert margin was less than 100 km. During the last two glacial periods, eolian sand was directly deposited at Jingbian, indicating a further southward desert shift. The Jingbian sand-sized particle record clearly demonstrates that, superimposed on the glacial-interglacial oscillations, the Mu Us Desert experienced signifi cant expansion at ~2.6, ~1.2, ~0.7 and ~0.2 Ma, directly implying a stepwise southward retreat of the monsoon rainfall belt, associated with a complementary reduction in summer monsoon strength, in the past 3.5 Ma. This evolutionary pattern may be causally linked to the Plio-Pleistocene increase in global ice volume, as shown in the marine oxygen isotope record (Fig. 3E). REFERENCES Ding Z.L., Sun, J., and Liu, T., 1999: Stepwise advance of the Mu Us desert since late Pliocene: Evidence from a red clay-loess record, Chin. Sci. Bull. 44:1211–1214. Ding Z.L., Derbyshire, E., Yang, S.L., Yu, Z.W., Xiong, S.F. and Liu, T.S., 2002: Stacked 2.6-Ma grain size record from the Chinese loess based on fi ve sections and correlation with the deep-sea δ18O record, Paleoceanography 17, 1033, doi: 10.1029/2001PA000725. Pye K., 1987: Aeolian Dust and Dust Deposits, Academic Press, London. Shackleton N.J. and Pisias, N.G., 1985: Atmospheric carbon dioxide, orbital forcing, and climate, in: E.T. Sundquist, W.S. Broecker (Eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations, Archean to Present, Geophysical Monograph Series 32, American Geophysical Union, Washington, DC, pp. 303–317. Shackleton N.J., Berger, A., and Peltier, W.R., 1990: An alternative astronomical calibration of the lower Pleistocene timescale based on ODP site 677, Trans. R. Soc. Edinburgh Earth Sci. 81: 251–261.